Deacetylation, of cellulose acetates

Enzymes with the same nominal specificity, such as acetyl xylanesterases, have different specificities in the deacetylation of cellulose acetate, according to their CE Family. This parallels, but in a more rational fashion, the use of lipases by synthetic chemists to deprotect sugar acetates regioselectively, or, in non-aqueous suspension, to acylate them selectively however, in these cases, the choice of lipase is purely empirical. 

Shrinkage, tackiness and increased brittleness due to the migration and subsequent evaporation of plasticiser from between the cellulose acetate chains, is also a frequent cause of degradation. The degradation of some plasticisers has been shown to increase acidity of cellulose acetate-containing materials. Triphenyl phosphate, used as a plasticiser for cellulose acetate since the 1940s, decomposes to form diphenyl phosphate and phenol. Diphenyl phosphate is a strong acid so it is likely to accelerate the deacetylation of cellulose acetate. 

C. Altaner, B. Saake, M. Tankanen, J. Eyzaguirre, C. B. Faulds, P. Biely, L. Viikari, M. Siika-aho, and J. Puls, Regioselective deacetylation of cellulose acetates by acetyl xylan esterases of different famihes, J. Biotech., 105 (2003) 95-104. 

Physicochemical studies of the solution properties of amylose benzoate by light-scattering, osmometric, and viscosimetric techniques showed that the molecules behave as coils. Factors affecting the rate of dissolution of cellulose acetate phthalate in aqueous solution are important from the point of view of use as an enteric-coating material. The removal of the benzoyl group during methanolysis is slow and concurrent with the methanolysis, " whereas deacetylation precedes methanolysis. 

The most prominent cellulose ester produced on the industrial scale is cellulose acetate. The reaction is usually performed with acetic anhydride and with sulfuric acid as a catalyst. To minimize heterogeneities, acetylation is allowed to run nearly to completion, and subsequently partial ester hydrolysis is initiated by the addition of water until a desirable solubility is achieved that corresponds to a DS of about 2.5. Such higher acyl homologues as propanoyl or butanoyl exhibit more thermoplastic properties. Many specialized esters such as chiral (-)-menthyloxyacetates, furan-2-carboxylates, or crown-ether-containing acylates have been prepared on the laboratory scale and characterized by NMR spectroscopy. Various procedures have been applied, using anhydrides and acyl chlorides as acylating agents in combination with such bases as pyridine, 4-dimethylaminopyridine (DMAP), or iV,iV -carbonyldi-imidazole. The substitution pattern of cellulose acetates has also been modified by postchemical enzymatic deacetylation. Cellulose 6-tosylates have been used as activated intermediates for nucleophihc substitution to afford 6-amino-6-deoxy, 6-deoxy, or 6-deoxy-6-halo-celluloses. ... 

Photodegradation of cellulose acetate fibres by u.v.-irradiation in vacuo at 77 or 293 K resulted in deacetylation and chain-scission. ... 

Pure crystals of cellulose IV have been prepared by deacetylation and precipitation of cellulose acetate of a low degree of polymerization at ISO— 160 C. At temperatures between 90 and 150 °C, hybrid crystals are obtained having cellulose II and cellulose IV, domains cocrystallizing in syntaxy. When a solution leading to cellulose IV was seeded with native cellulose fibrils, a shish-kebab structure of microfibi s with cellulose II lamellae was obtained. 

Starting cellulose, prepared by deacetylation of commercial, medium viscosity cellulose acetate (40.4% acetyl content). 

Acetone-soluble cellulose acetate is prepared by deacetylating cellulose triacetate. The product formed directly is unsatisfactory. Thus the distribution of free hydroxyls and acetate groupings is of primary importance. Cramer and Purves118(b) studied the distribution by tosylation and found that the acetyl removal from primary and secondary hydroxyl groups occurs at approximately the same rate, but that the number of... 

The complete hydrolysis of the soluble cellulose derivative required gradual addition of water and, preferably, acid. Water can act on cellulose acetate hydrogensulfate in three ways (a) saponification of sulfate groups, (b) saponification of acetate groups, and (c) hydrolysis of glycosidic bonds. Reaction (a) is most rapid and occurs under the mildest conditions. As Table II shows, the product is not any more water soluble (Entries 1-4). Under more severe conditions, the deacetylation (b) has proceeded to water-soluble products, the total amount of dissolved carbohydrates corresponding to theory (Entries 5-10). How-... 

Cellulose acetate is usually produced by the so-called solution process with exception of the fully acetylated end product (triacetate). In the solution process the pulp is first pretreated with acetic acid in the presence of a catalyst, usually sulfuric acid. The purpose of this activation step is to swell the fibers and increase their reactivity as well as to decrease the DP to a suitable level. Acetylation is then performed after addition of acetic anhydride and catalytic amounts of sulfuric acid in the presence of acetic acid. After full acetylation the final triacetate obtained is dissolved. This "primary" acetate is usually partially deacetylated in aqueous acetic acid solution to obtain a "secondary" acetate with a lower DS of about 2 to 2.5. 

Several classes of polymeric materials are found to perform adequately for blood processing, including cellulose and cellulose esters, polyamides, polysulfone, and some acrylic and polycarbonate copolymers. However, commercial cellulose, used for the first membranes in the late 1940 s, remains the principal material in which hemodialysis membranes are made. Membranes are obtained by casting or spinning a dope mixture of cellulose dissolved in cuprammonium solution or by deacetylating cellulose acetate hollow fibers [121]. However, polycarbonate-polyether (PC-PE) block copolymers, in which the ratio between hydrophobic PC and hydrophilic PE blocks can be varied to modulate the mechanical properties as well as the diffusivity and permeability of the membrane, compete with cellulose in the hemodialysis market. 

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